rcutorture: Read CPU ID for decoration protected by both reader types

Currently, rcutorture_one_extend() reads the CPU ID before making any
change to the type of RCU reader.  This can be confusing because the
properties of the code from which the CPU ID is read are not that of
the reader segment that this same CPU ID is listed with.

This commit therefore causes rcutorture_one_extend() to read the CPU
ID just after the new protections have been added, but before the old
protections have been removed.  With this change in place, all of the
protections of a given reader segment apply from the reading of one CPU ID
to the reading of the next.  This change therefore also allows a single
read of the CPU ID to work for both the old and the new reader segment.
And this dual use of a single read of the CPU ID avoids inflicting any
additional to heisenbugs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

kernel/rcu/rcutorture.c

@@ -271,12 +271,12 @@ struct rt_read_seg {
 	unsigned long rt_delay_us;
 	bool rt_preempted;
 	int rt_cpu;
+	int rt_end_cpu;
 };
 static int err_segs_recorded;
 static struct rt_read_seg err_segs[RCUTORTURE_RDR_MAX_SEGS];
 static int rt_read_nsegs;
 static int rt_read_preempted;
-static int rt_last_cpu;
 
 static const char *rcu_torture_writer_state_getname(void)
 {
@@ -1922,6 +1922,7 @@ static void rcutorture_one_extend(int *readstate, int newstate, bool insoftirq,
 				  struct torture_random_state *trsp,
 				  struct rt_read_seg *rtrsp)
 {
+	bool first;
 	unsigned long flags;
 	int idxnew1 = -1;
 	int idxnew2 = -1;
@@ -1930,12 +1931,11 @@ static void rcutorture_one_extend(int *readstate, int newstate, bool insoftirq,
 	int statesnew = ~*readstate & newstate;
 	int statesold = *readstate & ~newstate;
 
+	first = idxold1 == 0;
 	WARN_ON_ONCE(idxold2 < 0);
 	WARN_ON_ONCE(idxold2 & ~RCUTORTURE_RDR_ALLBITS);
 	rcutorture_one_extend_check("before change", idxold1, statesnew, statesold, insoftirq);
 	rtrsp->rt_readstate = newstate;
-	if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU))
-		rtrsp->rt_cpu = raw_smp_processor_id();
 
 	/* First, put new protection in place to avoid critical-section gap. */
 	if (statesnew & RCUTORTURE_RDR_BH)
@@ -1957,6 +1957,14 @@ static void rcutorture_one_extend(int *readstate, int newstate, bool insoftirq,
 	rcutorture_one_extend_check("during change",
 				    idxold1 | statesnew, statesnew, statesold, insoftirq);
 
+	// Sample CPU under both sets of protections to reduce confusion.
+	if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU)) {
+		int cpu = raw_smp_processor_id();
+		rtrsp->rt_cpu = cpu;
+		if (!first)
+			rtrsp[-1].rt_end_cpu = cpu;
+	}
+
 	/*
 	 * Next, remove old protection, in decreasing order of strength
 	 * to avoid unlock paths that aren't safe in the stronger
@@ -2178,8 +2186,6 @@ static bool rcu_torture_one_read(struct torture_random_state *trsp, long myid)
 	}
 	if (cur_ops->reader_blocked)
 		preempted = cur_ops->reader_blocked();
-	if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU))
-		rt_last_cpu = raw_smp_processor_id();
 	rcutorture_one_extend(&readstate, 0, myid < 0, trsp, rtrsp);
 	WARN_ON_ONCE(readstate);
 	// This next splat is expected behavior if leakpointer, especially
@@ -3634,8 +3640,13 @@ rcu_torture_cleanup(void)
 					err_segs[i].rt_delay_jiffies);
 				firsttime = 0;
 			}
-			if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU))
-				pr_cont(" CPU %-2d", err_segs[i].rt_cpu);
+			if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU)) {
+				pr_cont(" CPU %2d", err_segs[i].rt_cpu);
+				if (err_segs[i].rt_cpu != err_segs[i].rt_end_cpu)
+					pr_cont("->%-2d", err_segs[i].rt_end_cpu);
+				else
+					pr_cont(" ...");
+			}
 			if (err_segs[i].rt_delay_ms != 0) {
 				pr_cont(" %s%ldms", firsttime ? "" : "+",
 					err_segs[i].rt_delay_ms);
@@ -3666,8 +3677,6 @@ rcu_torture_cleanup(void)
 		}
 		if (rt_read_preempted)
 			pr_alert("\tReader was preempted.\n");
-		if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_LOG_CPU))
-			pr_alert("\tReader last ran on CPU %d.\n", rt_last_cpu);
 	}
 	if (atomic_read(&n_rcu_torture_error) || n_rcu_torture_barrier_error)
 		rcu_torture_print_module_parms(cur_ops, "End of test: FAILURE");